Hi Mike,
As Janet has pointed out, you have given us very little info to work on.
If I were a betting man I would bet $10 that you are fitting a polynomial
and copying the trendline parameters to see if you get close to the original
data. Try increasing the number of digits displayed by the trendline
equation. Better yet, use LINEST to put the parameters into cells and use
these values (see www.stfx.ca/people/bliengme/ExcelTips)
As Cassius said" The fault, dear Brutus, is not always with Excel"
Best wishes
Bernard
The trendline with unrounded equations will be more accurate than LINEST
for numerically challenging problems.
The other common problem people have with the chart trendline (besides
failing to show enough figures) is using a "Line" Chart instead of an
"XY (Scatter)" chart. The "Line" chart calculations ignore the x-values
that you supply, substituting 1,2,3,... and only uses supplied x's as
non-arithmetic labels.
In Excel versions prior to 2003, the trendline with unrounded equations
will be more accurate than LINEST for numerically challenging problems.
The other common problem people have with the chart trendline (besides
failing to show enough figures) is using a "Line" Chart instead of an
"XY (Scatter)" chart. The "Line" chart calculations ignore the x-values
that you supply, substituting 1,2,3,... and only useing supplied x's as
non-arithmetic labels.
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